Binder composition for use in three dimensional printing

ABSTRACT

A binder composition for three dimensional printing of parts is disclosed which is stable during storage and passage through a printhead, yet able to gel under the conditions existing in a powder bed. The binder composition comprises colloidal silica, a catalyst able to promote gelation of the composition when the composition is below a predetermined pH value, and a base able to maintain the pH of the composition above the predetermined value at which the composition gels. Preferably, the catalyst is polyethylene glycol or another ethylene oxide-derived polymer, and the base is triethanolamine. Upon impact with a powder bed, the pH of the binder composition is reduced, as by adding an acid such as citric acid to the powder, thereby causing the binder to gel in the powder.

GOVERNMENT RIGHTS

The U.S. Government has certain rights in this invention pursuant toContract No. 9215728-DDM, awarded by the National Science Foundation.

This is a divisional of application Ser. No. 08/581,319 filed on Dec.29, 1995. Now U.S. Pat. No. 5,660,621.

FIELD OF THE INVENTION

This invention relates to three dimensional printing and moreparticularly to binders used in three dimensional printing.

BACKGROUND OF THE INVENTION

Three dimensional printing is a process for manufacturing tooling andprototype parts in a layered fashion. A powdered material is depositedin a layer and bound in selected regions, defined by a computer model ofthe part, by a liquid binder material which is applied to the layer ofpowdered material using an ink-jet printhead. These steps are repeatedlayer by layer to build up the part. Commonly used powders includealuminum oxide, zirconium silicate, fused silica, stainless steel,tungsten, and silicon carbide. Colloidal silica is primarily used as thebinder for the ceramics and Acrysol® latex emulsion for the metals.After printing of all the layers, the part is treated in a mannerappropriate to the part and the materials chosen, for example bysintering, and unbound powder is removed. The process is shown generallyin FIG. 1. See, e.g., U.S. Pat. No. 5,204,055.

To form mechanically strong bonds between powder particles, 6 to 10% ofthe material comprising the finished part must be in the form of weldsbetween powder grains. If the welds are composed of adhesive dispensedby the printhead, then the liquid binder must contain approximately thesame percentage (6 to 10% by volume) of solids. Few inorganic substancesare soluble to this degree. Thus, the solids carried by the binder aregenerally dispensed as a dispersion of solid particles in a liquidcarrier, a colloid.

The binder is often stored for a few months on the shelf or in thereservoir. The binder is usually driven from the reservoir by gaspressure and passes through a number of fittings, tubing, and filtersbefore reaching the printhead. Most binder passing through the printheadnozzles lands in the collection gutter or catcher and is carried off bya secondary fluid pumping system. The binder is often recycled,particularly with multiple jet printheads.

As the printhead is swept over the powder bed, the binder is formed intoa stream of droplets which impact the powder bed and overlap oneanother, thereby merging to from a linear strip having a cylindricalcross-section across the powder. Once the liquid comes substantially torest in the powder, the primary driving force for further movement iscapillary attraction which can exert its influence until some mechanismcauses the binder solids to be immobilized, as by drying the binder, forexample, by heating or directing an air flow over the layer. FIG. 2illustrates the sequence from jet impact to immobilization.

Printhead failure mechanisms include drying of the binder on the nozzleand the clogging of the nozzle by solid material in the binder feed.Large (50 μm) solid particles can find their way into the binder if thesol dries on tube fittings when connections are broken, and they canalso form by gradual flocculation of the sol if its stability is notsufficiently high.

SUMMARY OF THE INVENTION

The present invention provides a binder composition for use in threedimensional printing of parts which becomes immobilized in the printedregion of a powder layer and does not migrate to unprinted regions. Thebinder includes a catalyst which controls the immobilization in thepowder. The invention also relates to a process for forming parts usingthree dimensional printing in which the binder immobilization process iscontrolled.

Specifically, the active ingredient of the binder composition iscolloidal silica, preferably comprising amorphous spherules dispersed ina liquid medium such as distilled water to form a sol. After printingonto a layer of ceramic powder, the colloidal silica forms a gel beforedrying in the powder bed. The binder catalyst is polyethylene glycol oranother ethylene oxide-derived polymer. This catalyst promotes gelationif the pH of the binder composition is below a predetermined value,approximately 7.5 for colloidal silica. Accordingly, to precludegelation during storage and passage through an ink jet printhead, thecomposition includes a base substance which maintains the pH above thepredetermined value. The preferred base, triethanolamine, maintains thepH of colloidal silica approximately between 9 and 9.5, which is withinthe range of maximum stability for the colloidal silica and is mostcompatible with the stainless steel and nickel printhead components.After printing, the pH is reduced below the predetermined value, therebycausing the binder to gel in the powder. In the preferred embodiment,the pH is reduced by adding an acid, preferably citric acid, to thepowder. The pH can be reduced by other mechanisms, however, such as byapplying gaseous CO₂ to the powder after printing.

The binder composition and related process are particularly useful forprinting ceramic shell molds for metal casting.

DESCRIPTION OF THE DRAWINGS

The invention will be more fully understood from the following detaileddescription taken in conjunction with the accompanying drawings inwhich:

FIG. 1 is a schematic illustration of a prior art three dimensionalprinting process; and

FIG. 2 is a schematic illustration of the sequence of droplet impact toimmobilization.

DETAILED DESCRIPTION OF THE INVENTION

A colloidal binder composition must be a well-dispersed, stable fluid towithstand prolonged storage, must be inert with respect to the printheadmaterials, and must have a sufficient volume fraction of solids. Theviscosity must-be low enough that it can be forced through the nozzle atthe desired rate. It must be stable under moderate shear in tubing,filtration, and the pumping mechanism and with respect to the largetransient shear rate that is encountered in jet formation, avoidexcessive wear to the orifice when passing through it, and present astable meniscus to the outside surface of the nozzle. The compositionmust be miscible with the rinse solution and should dry slowly and beeasy to clean up. Once the liquid encounters the powder bed, thestability requirement is reversed: the solids from the suspension mustdeposit on powder grains and join them together. Thus, the dispersionmust be stable, yet not so stable that it fails to flocculate or gel tostick to powder grains under the conditions that exist in the powderbed.

A binder composition according to the present invention comprisescolloidal silica dispersed in a fluid dispersing medium, a catalyst topromote gelation of the binder composition when the pH of thecomposition is below a predetermined value, and a base able to maintainthe pH of the binder composition above the predetermined value at whichthe binder composition gels.

The colloidal silica is an aqueous dispersion of spherical particles ofamorphous silica, also called silica sol, commercially available inuniform particle sizes ranging from 5 nm to 100 nm. Nyacol 9950,available from Eka-Nobel, has been found to be suitable. It has aparticle size of 100 nm and a solids loading of 50% by weight (33% byvolume). Larger particle sizes are preferable, since they permit moresolids to be loaded into the suspension without losing stability. TheNyacol 9950 should be prefiltered to 5 μm to remove any large inclusionswhich may be present.

The fluid dispersing medium is preferably distilled water. TheNyacol/water balance is typically maintained with the silica atapproximately 17.5% by volume. The colloidal silica should be present inthe water to at least 6 to 100% by volume (12 to 20% by weight) to forma bond between powder. The maximum amount of silica is preferred.However, for ease of handling, the suspension is generally diluted tobelow 20% by volume (32% by weight).

The binder composition also includes a base to maintain the pH above thepredetermined pH value to prevent premature coagulation. For colloidalsilica, the base should be chosen to hold the pH between 9 and 10, whichis the range of maximum stability of the colloidal silica and is mostcompatible with the stainless steel and nickel printhead components. Thequantity of the base is determined by the amount of citric acid normallypresent in the powder: the amount must be sufficient to allow the binderpH to drop to 7 or lower in the powder bed. The quantity of the basemust also be sufficient to overwhelm the effect of impurities that maybe picked up during storage. For example, CO₂ in air is mildly acidicand will cause the pH of a very weak solution to change.

Triethanolamine (TEA, 2-2'-2" Nitrilotriethanol), a weak base, has beenfound to be suitable. Preferably, a concentration of approximately 2% byweight of the total composition is sufficient to hold the pH of thebinder at about 9 to 9.5. TEA is also advantageous, because it does notprecipitate or evaporate when the binder dries. Therefore, the driedbinder still has a high pH and is easy to redisperse, which aids inclean up if the binder dries on the printhead or catcher.

Other bases which do not react adversely with the other components canalso be used. For example, tetramethylammonium hydroxide (TMAH),ammonia, and morpholine were tested and found to be suitable. Ammonia,being a stronger base than, for example, TEA, is able to raise the pH ofthe binder to almost 10 at a concentration of 0.5% by weight. Ingeneral, a concentration of approximately 0.5% by weight of a suitablebase should be sufficient. Some strong inorganic base, such as sodiumhydroxide, should have their concentrations kept to a minimum, sincelarger concentrations can destabilize the colloid and impair themechanical properties of the finished ceramic.

The coagulation ability of the bases TEA, TMAH, ammonia, and morpholinewas tested by measuring the light transmission through samples ofcolloidal silica mixed with varying amounts of each of the bases towhich varying amounts of citric acid were added. TMAH had the greatestcoagulating ability at the lowest concentration, as low as 0.01 mol/l atpH 5.5. Both morpholine and TEA had similar coagulating ability, at aconcentration of approximately 0.05 mol/l at pH 5.5. Ammonia had theleast coagulating ability, at a concentration of approximately 0.2 mol/lat pH 5.5.

The age stability of the four bases was also tested by heating thesamples of binder and base to simulate storage for two months. TMAHexhibited the largest drop in pH. The pH of the mixtures with ammoniaalso declined at the lower concentrations of ammonia. Morpholine and TEAexhibited relatively no change in pH.

When binder dries, it is desirable that the base not evaporate out ofthe gel. This permits the gel to be redispersed when water is addedback, which greatly aids in cleaning of the printhead components andprevents the formation of a solid crust on dried tube fittings. Bothmorpholine and TEA are liquids at room temperature. Morpholine has aboiling point of 130° C., and TEA has a boiling point of 278° C. Thehigher boiling point of TEA suggests it would evaporate the least. Thus,TEA appears to be a preferred base to raise the pH of the binder.

With TEA, a catalyst is required to coagulate the binder in the lower pHenvironment of the powder bed. The preferred binder catalyst ispolyethylene glycol (PEG) or another ethylene oxide-derived polymer.This catalyst promotes gelation if the pH of the binder composition isbelow approximately 7.5. The optimum molecular weight of the PEGcomponent is at least 14,000 and preferably approximately 20,000 (or adegree of polymerization of at least 400 to 600). The higher themolecular weight, the less is needed to coagulate the binder. However,molecular weights in excess of 35,000 inhibit the breakup of the jetexiting the printhead nozzle.

Silica below a pH of 7.5 has a substantial amount of hydrogen ionsadsorbed onto it. The PEG has a lot of oxygen in its ether bonds whichare attracted to the hydrogen in the silica. The phenomenon is fairlyreversible; thus, if the binder pH is increased, it can be mostlyredispersed. Only a small amount of PEG is needed to gel the binder.Amounts as small as 0.001% by weight of the total composition or 100 ppmare sufficient. Typically, 0.1% is used, although the amount can begreater. However, too much PEG causes the silica to become stable at allpHs and defeats the gelation mechanism. This upper limit depends on theparticle size and concentration of the colloid, since it corresponds tothe point at which the silica particles become completely coated withthe polymer.

It is desirable to add a humectant to retard drying of the bindercomposition. This slows down buildup of the binder in the printheadcatcher and drying at the nozzle, enabling the printhead to be stoppedand restarted within a few minutes without difficulty. Propylene glycol(1-2 propanedial) is the preferred humectant. Ethylene glycol may alsobe used, although it is toxic. Glycol also seems to soften the bindergel which aids cleaning of the printhead components and may reducedistortion during printing. As a cosolvent, its presence probablyimproves the binder's solubility for organic impurities which cancontribute to jet instability. An amount of approximately 5% by weightof the total composition has been found to be suitable to allow theprinthead to be restarted after remaining idle and filled with binderfor about one hour, which is usually adequate time for servicing, and toprevent drying on the waste gutter.

Experiments were conducted on the time it took a nozzle to clog afterstanding with a binder composition containing varying amounts ofethylene glycol. For compositions with less than 10% by volume glycol,the time to clog ranged from 2 to 16 minutes. For 10% glycol, the timeto clog ranged from 30 minutes to 1 hour. For 25% glycol, the time toclog was several days, and for 50% glycol, the nozzle remained uncloggedafter 19 days. Since the printhead components can generally be servicedwithin an hour, 10% by volume glycol should generally be sufficient.Other experiments indicated that ethylene glycol has no significanteffect on the coagulation of colloidal silica by TEA.

It is also desirable to add a cosolvent to keep organic impurities fromprecipitating onto the printhead components. For example, an ionexchange resin is an impurity that comes in the Nyacol 9950. Two percentby weight of butyl carbitol (diethylene glycol monobutyl ether,1-(2-Butoxyethoxy) ethanol) has been found useful in this regard.However, the butyl carbitol reduces the surface tension somewhat andleads to foaming, which can impair the printhead. Thus, the butylcarbitol can be omitted. Alternatively, the antifoaming agent Antifoam1500 available from Dow Corning, which is an emulsion containingmethylated silica and polydimethylsiloxane, can be added.

A pH indicator may be added to the binder composition to provide avisual indication of the pH of the binder composition. It can be used tomonitor the condition of the binder during storage and recycling and toassess the acid content of the powder bed. A suitable pH indicator isthymol blue, which is blue above pH 8 and yellow at pH 7 or below. Ifcitric acid is used in the powder bed to reduce the pH to cause thesilica to gel, the thymol blue should cause the printed layers to beyellow or red. If the printed layers are green or blue, then not enoughcitric acid is present. The ideal mix is yellow with red dots around thecitric acid granules. Thymol blue may be added in amounts sufficient toprovide a visual indication. For example, approximately 0.125 g/literprovides a suitable blue color.

Citric acid is added to the powder layer to trigger coagulation of thebinder. The acid must be sufficiently concentrated so that it lower thebinder pH below the gelation threshold of about 7.5. It is desirable touse two or three times the minimum amount in the powder, so that somesegregation of the citric acid in the powder can be tolerated and toincrease the rate of gel growth. A citric acid concentration of 0.2 to0.5% by weight has been found to be sufficient. This acid in combinationwith the base TEA possess long-term stability after formulation prior toprinting, can be easily burned out of the printed material, arenonvolatile at ambient conditions, and do not pose any severe healthhazard.

To make the binder composition according to the preferred embodiment,the following components are combined and mixed thoroughly to dissolvethe solids:

    ______________________________________                                        distilled water       385.9 cc (385.9 g)                                      propylene glycol      58.4 cc (65.1 g)                                        triethanolamine       21.7 cc (24.4 g)                                        diethylene glycol monobutyl ether                                                                   12.6 cc (12.2 g)                                        polyethylene glycol   1.0 g                                                   thymol blue           0.5 g                                                   ______________________________________                                    

To this mixture, 525.0 cc (735.0 g) of Nyacol 9950 are added. The silicaappears to flocculate upon-mixing. According, this mixture should beallowed to stand for a time or should be filtered by pumping in a closedcircuit through a 5 μm filter for a time to redisperse the flocs.

The specific gravity of this mixture is 1.21 for 17.5 vol. % silica. ThepH is between 9 and 9.5. The viscosity is approximately 2 to 3 cP (0.002to 0.003 Pa-s). The surface tension is 54 dyn/cm (0.054 Pa-m).

Other methods to reduce the pH of the binder composition to causegelation are possible. Far example, gaseous CO₂ can be applied to eachlayer of the powder after printing of the binder.

The invention is not to be limited by what has been particularly shownand described except as indicated by the appended claims.

I claim:
 1. A process for forming a three dimensional part,comprising:building up the part in layers, the formation of each layercomprising:(a) providing a layer of powder in a bed; (b) providing abinder composition comprising:a fluid dispersing medium; silica in asolid particulate form dispersed in the fluid dispersing medium; acatalyst able to promote gelation of the binder composition when thebinder composition is below a predetermined pH value; and a base able tomaintain the pH of the binder composition above the predetermined valueat which the binder composition gels; and (c) selectively depositingthrough a print head a binder composition onto the powder layer; andaltering the pH of the binder composition in the powder to below thevalue at which the binder composition gels, whereby the bindercomposition becomes immobilized in the powder.
 2. The process of claim1, wherein the pH of the binder composition in the powder is altered byproviding an acid to the powder prior to the step of selectivelydepositing the binder composition to the powder.
 3. The process of claim1, wherein the acid comprises citric acid at a concentration ofapproximately 0.5 percent by weight of powder.
 4. The process of claim1, wherein the pH of the binder composition in the powder is altered byapplying gaseous CO₂ to the binder composition and the powder after thestep of the selectively depositing the binder composition.
 5. Theprocess of claim 1, wherein the powder comprises aluminum oxide orzirconium silicate.
 6. The process of claim 1, wherein the fluiddispersing medium comprises distilled water.
 7. The process of claim 1,wherein the base substance comprises triethanolamine.
 8. The process ofclaim 1, wherein the base substance is able to maintain the pH of thebinder composition above 7.5.
 9. The process of claim 1, wherein thebase substance is able to maintain the pH of the binder compositionbetween 9 and 9.5.
 10. The process of claim 1, wherein the silica isamorphous.
 11. The process of claim 1, wherein the silica comprises atleast 10 percent by volume of the binder composition.
 12. The process ofclaim 1, wherein the silica comprises at least 17.5 percent by weight ofthe binder composition.
 13. The process of claim 1, wherein the polymeris made from an ethylene oxide.
 14. The process of claim 13, wherein thepolymer made from an ethylene oxide comprises polyethylene glycol. 15.The process of claim 13, wherein the polymer made from an ethylene oxideis provided at a concentration of at least 100 ppm.
 16. The process ofclaim 13, wherein the molecular weight of the polymer made from anethylene oxide is between 400 and 35,000.
 17. The process of claim 13,wherein the concentration of the polymer made from an ethylene oxide isapproximately 0.1 percent by weight.
 18. The process of claim 1, furthercomprising a pH indicator.
 19. The process of claim 18, wherein the pHindicator comprises thymol blue.
 20. The process of claim 1, wherein thebase substance comprises an organic base.
 21. The process of claim 1,wherein the base substance comprises triethanolamine,tetramethylammonium hydroxide, ammonia, or morpholine.